Signal processing system and method

ABSTRACT

An adaptive signal processing system ( 800 ) comprises a plurality of receiving elements ( 802   a - n ), a plurality of analogue to digital converters (ADC&#39;s) ( 806   a - n ) and a digital signal processor ( 808 ). Each of the receiving elements ( 802   a - n ) is arranged to receive an incoming signal and has an ADC ( 806   a - n ) connected thereto. Each ADC *( 806   a - n ) is arranged to convert a first portion of respective incoming signal into a digital form at a sampling rate less than the temporal Nyquist rate of the incoming signal. The signal processing ( 808 ) means is arranged to calculate complex weighting coefficients to be applied to a second portion of the incoming signal using the digitised first portion of the incoming signals. A method of calculating complex weighting coefficients is also described.

This invention relates to a signal processing system and method. Moreparticularly, but not exclusively, the invention relates to a system andmethod for adaptive signal processing for beamforming and signaldetecting.

Phased array antennas where energy impinges upon an array aperture of aplurality of radiating/receiving elements are well known. In a phasedantenna an incoming wavefront is subject to weighting by a complexfunction at each individual radiating element. The purpose of theweighting is to compensate for the phase shift experienced by thewavefront in traversing the distance between radiating elements in orderthat the contributions from the wavefront can be summed by a beamformer.It is these weightings that effectively steer phased array antennas, aschanging the weightings for antennas elements changes the direction towhich the antenna is tuned.

The weighting procedure can be used to spatially filter incoming signalsof the same frequency by spatially rejecting interfering signals. Thisspatial rejection of signals, or nulling, is achieved by weighting asignal from the interfering direction such that the weighted componentsof the signal are in antiphase and when summed given a zero, null,amplitude. Nulling has the attendant difficulty that the location ofeach user must be known in order to calculate the correct weightings inorder to be able to place the interfering signals components inantiphase, i.e. it requires a priori knowledge of the system that willnot always be available. Therefore adaptive phased array antennas samplein the temporal domain in order to overcome this problem.

In a true adaptive (smart) antenna a computation is carried out in orderto ascertain the weightings that must be applied in order to fulfil agiven criteria, for example a given steering direction.

In a prior art analogue smart antenna as typically used intelecommunication applications, as shown in FIG. 3, respective portionsof a signal received by respecting radiating elements are passed to anadaptive processor along with an error signal. The error signal is thedifference between an output from a beamformer, the output containing awanted signal and unwanted signals from interferers, and a desired,training, signal. The adaptive processor performs a least mean squaredanalysis in order to minimise this error by varying the weightings. Thissmart antenna arrangement has the problem that the desired signal mayonly be known for a short period of time and the signal changes as theradiator moves (and possibly as changes in the environment through whichthe wave is propagated occur—e.g. changing reflection). Thisnecessitates the frequent use of training sequences where a known signalis radiated and any error between the received signal and a referencemust be corrected. The correction conversion is applied to the nextsignal and subsequent signals which are expected to have reasonablesignal coherence. For example, in GSM training sequences are emittedthat account for 23% of the data stream.

Prior art adaptive antennas which are used in radar systems, as shown inFIG. 4, employ a similar technique whereby signals received by eachradiating element are sampled by an adaptive processor which is suppliedwith a steering vector. The steering vector comprises the magnitude andphaseshifts required to steer a beam in a given direction (or to steerthe sensitivity response pattern of the antenna in a given direction).

The adaptive algorithm executed by the adaptive processor typicallyminimises the power at the beamformer output in order to null thecontributions from interferers.

The use of a known steering vector is possible as the radar is scanningand therefore the direction of radiation/arrival is known. Thebeamformer must be able to process the incoming signals at the temporalNyquist rate, typically 10's of MHz (i.e. the rate of change of temporalinformation within the sample). This is why the majority of beamformersare analogue as there is no Nyquist sampling constraint in analoguesignal processing. However, any digital processor used must also be ableto calculate weighting coefficients within the coherence time of thechannel.

The Nyquist sampling requirement applies to all-digital systems wherethe incoming signal is digitised prior to beamforming and in which boththe beamforming and calculation of the new weighting coefficents arecarried out by a digital processor.

There are also hybrid digital-analogue systems where the beamforming iscarried out by an analogue beamformer controlled by digital controllersthat are supplied with weighting coefficients calculated by a digitalprocessor. The Nyquist sampling requirement applies to the sampling ofthe analogue signal prior to its digitisation for calculating theweighting coefficients.

In a fully digitised scheme that samples in the temporal domain, asshown in FIG. 5, the signals from the radiating elements are digitisedby analogue to digital converters and passed to an adaptive processorthat calculates the weightings and passes them to a microprocessor wherethe weighting procedure is carried out. This arrangement has theattendant problem that due to the high bandwidths involved, for exampleradar signals have a typical bandwidth of >10 MHz, a very high samplingrate (10's of MHz, for example about 20 MHz) must be employed to fulfilthe temporal Nyquist requirement and prevent aliasing. High samplingspeeds require very fast and therefore very expensive ADC's.

In a prior art purely analogue antenna scheme, as shown in FIG. 6, theincoming signal from each radiating element is mixed with a lowerfrequency, phase controlled signal from a respective phase controlledlocal oscillator typically using a vector modulator. This imposes thephase of the local oscillator on the incoming signals at the beamformer.The problem in turning this arrangement into an adaptive antenna schemeis the necessity to sample the incoming sample at the requiredfrequencies. This is a significant technical challenge in the analogueregime.

It is an aim of at least one embodiment of the present invention toprovide an adaptive signal processing system that, at least partially,ameliorates at least one of the above-mentioned problems and/ordisadvantages, or that is an improvement over the prior art.

It is a further aim of at least one embodiment of the present inventionto provide a method of signal processing that, at least partially,ameliorates at least one of the above-mentioned problems and/ordisadvantages, or that is an improvement over the prior art.

According to a first aspect of the present invention there is providedan adaptive signal processing system comprising a plurality of receivingelements, a plurality of analogue to digital converters (ADC's) anddigital signal processing means; each of the receiving elements having arespective one of the ADC's connected thereto and being arranged toreceive a respective incoming signal, wherein each ADC is arranged toconvert a first portion of the respective incoming signals into adigital form at a sampling rate that is less than the temporal Nyquistrate of the incoming signal, and the signal processing means is arrangedto calculate complex weighting coefficients to be applied to respectivesecond portions of the respective incoming signals.

It has been appreciated, now, that it is possible to sample a signalusing sub-Nyquist sampling rates in a temporal domain whilst fulfillingthe Nyquist sampling criteria in the spatial domain. That is to say, therealisation that weighting coefficients are weighted to compensate forspatial domain factors and that things alter in the spatial domain a lotmore slowly than in the temporal domain, and that this means that it ispossible to have a sampling rate of a signal (for use in calculatingweighting functions), that is less, often very much less, than theNyquist sampling rate of a received signal that applies to its temporalinformation contact.

This means that it is possible to use lower power processors tocalculate the weighting coefficients (or take up less capacity of anygiven processor), and that the ADC converters need not have so fast asample-and-hold acquisition time.

Preferably the processor is arranged to utilise the digitised firstportion of the incoming signal in the calculation of the complexweighting coefficients

The system may include a second plurality of ADC's that are arranged tosample the second portion of the respective incoming signals at at leastthe temporal Nyquist sampling rate. The first plurality of ADC's may bearranged to sample the incoming signals prior to them reaching thesecond plurality of ADC's.

The processing means may be arranged to apply the complex weightingcoefficients to the second portion of the incoming signals.Alternatively, or additionally there may be a second digital processingmeans arranged to apply the complex weighting coefficients to the secondportion of the incoming signals. The first and second digital processingmeans may be distinct parts of a digital signal processing (DSP).Alternatively, the first and second digital processing means may besignal processing routines executed on a single DSP.

Each of the first plurality of ADC's may have a sample and hold unitcircuit typically with an acquisition time of 0.1 ns or less, or of theorder of 0.1 ns or less.

The system may include at least one plurality of phase changing means,each phase changing means being associated with a respective receivingelement, each of which may be connected to a digital control unit. Eachof the digital control units may be arranged to receive respectiveoutputs from the processing means that are indicative of the complexweighting coefficients. Each control unit may be arranged to supply adigital or analogue signal to a respective phase changing means suchthat the second portion of the respective fraction of the incomingsignal has its phase and/or amplitude varied by an amount correspondingto the complex weighting coefficient. The phase changing means maycomprise analogue components adapted to change the phase of a signal,the phase changing means being either under analogue control or digitalcontrol.

The ADC's may be arranged to sample respective signals after they havebeen passed through the phase changing means. The processing means maybe arranged to remove the phase and/or amplitude variations imposed uponthe first portions of the respective signals by the phase changingmeans. The second portions of respective signals may be the whole of therespective signals.

The phase changing means may include phase controlled local oscillatorand/or and may include a vector modulator, for example a diode.

The signal may have a frequency of the order of, or between any pair of,the following when sampled by the first plurality of ADC's: <20 MHz, 20MHz, 50 MHz, 125 MHz, 135 MHz, 145 MHz, 150 MHz, 500 MHz, 750 MHz, 1GHz, 1.25 GHz, >1.25 GHz.

The first plurality of ADC's may be arranged to sample the incomingsignal at a rate of the order of, or between any two of, the following:<20 kHz, 20 kHz, 40 kHz, 50 kHz, 60 kHz, 80 kHz, 100 kHz, >100 kHz.

According to a second aspect of the present invention there is provideda digital adaptive signal processing system comprising a plurality ofreceiving elements, first and second digital signal processing means,first and second pluralities of analogue to digital converters (ADC's);each of the receiving elements being arranged to receive an incomingsignal, the first plurality of ADC's being arranged to sample respectivefirst portions of the respective signals at a first sampling rate andthe second plurality of ADC's being arranged to sample a second portionof the respective signals at a second sampling rate, the second samplingrate being higher than the first sampling rate, wherein the firstdigital signal processing means is arranged to calculate complexweighting coefficients for each of the respective signals using theoutputs from the first plurality of ADC's and the second digital signalprocessing means is arranged to apply said weightings to said secondportions of said signals.

The first plurality of ADC's may be arranged to sample the first portionof the respective incoming signal at a rate below the temporal Nyquistrate, but at or above the spatial Nyquist sampling rate, of the incomingsignal. The second plurality of ADC's may be arranged to sample thesecond portion of the respective incoming signal at or above thetemporal Nyquist sampling rate of the incoming signal.

The first plurality of ADC's may be arranged to sample the incomingsignal prior to it reaching the second plurality of ADC's.

The first and second signal processing means may be different digitalsignal processors (DSP's). Alternatively, the first and second signalprocessing means may be distinct parts of a DSP. Alternatively, thefirst and second signal processing means may be signal processingroutines executed on a single DSP. The first signal processing means maybe arranged to communicate the complex weighting coefficients to thesecond signal processing means and the second signal processing meansmay be arranged to apply each complex weighting coefficient to a respectfirst portion of respective incoming signals.

The system may include at least one plurality of frequencydown-converting means each of which is associated with respectivereceiving elements and respective first ADC's may be arranged to samplethe first portion of each respective signal after it has passed throughrespective down-converting means.

The down converting means may be a frequency mixer.

The incoming signals may have a frequency of the order of, or betweenany pair of, the following once it has passed through the frequency downconversion means: <20 MHz, 20 MHz, 50 MHz, 125 MHz, 135 MHz, 45 MHz, 150MHz, 500 MHz, 750 MHz, 1 GHz, 1.25 GHz, >1.25 GHz.

The first plurality of ADC's may have a sample and hold circuit,typically with an acquisition time of the order of 0.1 ns or less.

The first plurality of ADC's may be arranged to sample the incomingsignal at a rate of the order of, or between any two of the following:<20 kHz, 20 kHz, 40 kHz, 50 kHz, 60 kHz, 80 kHz, 100 kHz, >100 kHz.

According to a third aspect of the present invention there is provided adigitally controlled analogue adaptive signal processing systemcomprising a plurality of receiving elements, at least one plurality ofphase changing means, a plurality of analogue to digital converters(ADC's), a digital signal processing means, and an analogue signalprocessing means; and in which each of the receiving elements isarranged to receive an incoming signal, each of the receiving elementsis arranged to feed a respective phase changing means, each ADC isarranged to sample the output from a respective phase changing means,the digital signal processing means is arranged to calculate respectivecomplex weighting coefficients to be applied to respective incomingsignals and to control the phase changing means so as to apply, in use,the respective weighting coefficients to the respective incoming signalsreceived by respective receiving elements.

Preferably the processor is arranged to utilise the digitised sample ofthe incoming signal in the calculation of the complex weightingcoefficients.

The system may include at least one plurality of phase changing means,each phase changing means being associated with a respective receivingelement. The system may include at least one plurality of frequencydown-shifting means, each down-shifting means being associated with arespective receiving element. At least one of each of the down shiftingmeans and phase changing means may be the same device.

Each of the phase changing means may be connected to a respectivedigital control unit. Each of the digital control units may be arrangedto receive respective outputs from the processing means that areindicative of the complex weighting coefficients. Each control unit maybe arranged to supply an analogue or digital signal to its respectivephase changing means such that the second portion of each fraction ofthe incoming signal has its phase and/or amplitude varied by an amountcorresponding to the complex weighting coefficient. The phase changingmeans are preferably adapted to operate in an analogue manner on thesignals it receives from their respective antennae elements.

The signal may have a frequency of the order of, or between any pair, ofthe following when sampled by the ADC's: <20 MHz, 20 MHz, 50 MHz, 125MHz, 135 MHz, 145 MHz, 150 MHz, 500 MHz, 750 MHz, 1 GHz, 1.25 GHz, >1.25GHz.

The ADC's may have a sample and hold circuit, typically with anacquisition time of 0.1 ns or less.

The ADC's may be arranged to sample the incoming signal at a rate of theorder of or between any two of the following: <20 kHz, 20 kHz, 40 kHz,50 kHz, 60 kHz, 80 kHz, 100 kHz, >100 kHz.

According to a fourth aspect of the present invention there is providedan adaptive signal processing system comprising a plurality of analogueto digital converters (ADC's) and digital signal processing means; eachof the ADC's being arranged to receive a respective incoming signal froma receiving element, wherein each ADC is arranged to convert a firstportion of the incoming signal into a digital form at a sampling rateless than the temporal Nyquist rate of the incoming signal, and thesignal processing means is arranged to calculate respective complexweighting coefficients to be applied to a second portion of the incomingsignals.

Preferably the processor is arranged to utilise the digitised firstportion of the incoming signal in the calculation of the complexweighting coefficients.

According to a fifth aspect of the present invention there is provided amethod of signal processing comprising the steps of:

-   -   i) receiving an analogue signal at a plurality of receiving        elements;    -   ii) sampling the signal at a sampling rate below the temporal        Nyquist rate of the signal;    -   iii) converting said samples into a digital form; and    -   iv) calculating an individual adaptive complex weighting        coefficient for the signals received at each of the plurality of        receiving elements digitally.

The method may include utilising the digitised samples in thecalculation of the complex weighting functions. The method may includeapplying each weighting coefficient to its respective signal.

The method may include sampling each signal at a sampling rate above thespatial Nyquist sampling rate of the signal. The method may includesampling the signal at a rate of the order of, or between any two of,the following: <20 kHz, 20 kHz, 40 kHz, 50 kHz, 60 kHz, 80 kHz, 100kHz, >100 kHz. The method may include down-shifting the frequency of thesignals, typically employing a down-shifting mixer, prior to samplingthe signals. The method may include down-shifting the frequency of thesignals to the order of, or between any pair of, the following: <20 MHz,20 MHz, 50 MHz, 125 MHz, 135 MHz, 145 MHz, 150 MHz, 500 MHz, 1 GHz, 1.25GHz, >1.25 GHz.

The method may include sampling the signals either before or afterpreviously calculated complex weighting coefficients have been appliedthereto. In the case where previous complex weighting coefficients havebeen applied to the signals the method may include removing therespective previously applied coefficients from the samples prior tocalculating the individual complex weighting coefficients.

The method may include applying each complex weighting coefficient toits respective signal in a digital processing means. Alternatively themethod may include applying each complex weighting coefficient to itsrespective signal by a digitally controlled analogue phase changingmeans. The method may include downshifting the frequency of and applyingthe complex weighting coefficient to each respective signal by means ofa single device.

According to a sixth aspect of the present invention there is provided amethod of producing weighting coefficients for adaptive beamformingcomprising undersampling analogue signals from antenna elements incomparison with a temporal Nyquist rate; converting the undersampledanalogue signals to digital signals in order to produce the weightingcoefficients.

Preferably the method include applying the weighting coefficients tosignals from the antennae elements.

Preferably the method includes receiving signals containing temporalinformation at a control processor at a first frequency and receivingsignals relating to the weighting coefficients at the control processorat a second frequency, the first frequency being significantly higherthat the second frequency. More preferably the first frequency is atleast about an order of magnitude higher than the second frequency, orat least about two orders of magnitude higher, or at least about threeorders of magnitude higher.

Preferably the method include applying the weighting coefficients tosignals containing temporal information in an analogue manner.Preferably the method includes undersampling analogue signals andconverting them to digital signals in order to produce the weightingcoefficients. The method may include generating the weightingcoefficients using digitally undersampled signals to control analoguecombination means to combine temporal signals with weightingcoefficients

According to a seventh aspect the invention comprises a signal sampleradapted to sample the signals from an array of antenna elements; aweighting generator adapted to generate from the signals from the signalsampler a weighting to be applied to signals received from an array ofantenna elements; and a signal combiner adapted to combine the signalsreceived from antenna elements with respective weightings from theweighting generator to produce modified antenna output signals; and abeamformer adapted to sample the modified antenna output signals at abeamformer rate and create a detected beam from there, and in which thesignal sampler is adapted to sample signals received at the antennaelements at a rate that is significantly slower than the beamformerrate.

Preferably the signal sampler rate is at least in order of magnitudeless than the beamformer sampling rate. Preferably the beamformer has aplurality of sample and hold devices which have a speed that issignificantly faster than sample and hold devices of the signal sampler,preferably at least an order of magnitude faster.

According to an eighth aspect of the present invention, there isprovided an adaptive signal processing system comprising a plurality ofreceiving elements, beamforming means, a plurality of analogue todigital converters (ADCs), digital processing means, and in which thesignal weighting means have a plurality of input channels and arespective plurality of output channels, the signal processing meansincluding a memory unit arranged to temporarily store a plurality ofpreviously calculated complex weighting coefficients, each of thereceiving elements being arranged to receive an incoming signal andbeing connected to a respective input channel, each of the ADCs beingarranged to sample an analogue signal directly from an output channel,and convert it into digital signals wherein the processing means isarranged to calculate new complex weighting coefficients using thedigitised signals and the previously calculated coefficients.

This system has the advantage, over the prior art beamformingarrangements discussed, that by sampling the incoming signals after thebeamformer the frequency of the sampled signals is greatly reducedcompared to the incoming signals, thus allowing lower speed, cheaper,ADCs to be used. It also allows a slower, cheaper, and/or uses less of aprocessor's processing power, to be used in calculating the newweighting coefficients as the beamformer reduces the frequency of theincoming signals prior to sampling. Therefore, the temporal Nyquistcriteria can be fulfilled at a lower sampling and processing rate.

The memory unit may be arranged to temporarily store the new complexweighting coefficients, typically by overwriting the previouslycalculated complex weighting coefficients. The processing means may bearranged to transfer the new coefficients to the signal weighting meansand the signal weighting means may be arranged to apply the newcoefficients to an incoming signal.

The signal weighting means may include a plurality of digital controlunits and/or respective analogue phase modulators. The digital controlunits may be arranged to receive respective new weighting coefficientsfrom the processing means and may be arranged to control respectivephase modulators so as to beamform the incoming signals, in response tothe new weighting coefficients, in use.

Each output channel may be arranged to be sampled by a coupling circuitor by tapping a portion of the signal from the output channel.

The signal weighting means may be arranged to receive incoming signalsat the order of any one of the following frequencies: 10 GHz, 1 GHz, 500HMz. The signal weighting means may be arranged to output signals at theorder of any one of the following frequencies: 100 MHz, 50 MHz, 10 MHz.

The signal weighting means may be arranged to downshift the frequency ofthe incoming signals, typically from the order of 1 GHz to the order of50 MHz.

The ADC's may be arranged to sample the signal after the signalweighting means and before a summation means.

The ADC's may be arranged to sample the signal at a rate between anypair of the following: <10 kHz, 10 kHz, 25 kHz, 30 kHz, 40 kHz, 50 kHz,75 kHz, 100 kHz, >100 kHz.

According to a ninth aspect of the present invention, there is provideda method of adaptive signal processing comprising the steps of:

-   -   (i) receiving an analogue signal;    -   (ii) downshifting the frequency of the signal;    -   (iii) beamforming the signal;    -   (iv) sampling the signal after beamforming;    -   (v) converting the signal sample into a digital signal; and    -   (vi) calculating a new complex weighting coefficient using the        digitised signal; using the digital signal and the previously        calculated complex weighting coefficients digitally.

The method may include controlling step (ii) using the previouslycalculated complex weighting coefficient.

The method may include receiving a plurality of signals and executingsteps (ii) to (v) for each respective signal, typically in parallel.

The method may include providing a beamformer in the form of an analoguephase modulator which may be arranged to be controlled by a digitalcontrol unit. The method may include providing a plurality of phasemodulators and digital control units.

The method may include accessing the previously calculated complexweighting coefficient from a memory unit in order to perform step (v).Preferably, the method includes any one, or combination of: subtracting,multiplying, adding, dividing, or non-linearly operating: from, with,to, by the previously calculated coefficient upon the digitised samplein order to perform step (v). The method may include storing the newcomplex weighting coefficient in the memory unit, preferably overwritingthe previously calculated coefficient with the new coefficients. Themethod may include providing processing means to calculate the newcomplex weighting coefficient.

The method may include passing a digital signal containing informationindicative of the new coefficient to the digital control unit. Themethod may include generating an analogue output from the digitalcontrol unit so as to control the phase modulator to execute step (ii).

The method may include processing the analogue signal prior tobeamforming. The processing may typically include any one, orcombination, of the following: downshifting frequency mixing,attenuation and/or phase modulation.

The method may include executing step (iii) of the method at between anypair of the following values: <10 kHz, 10 kHz, 25 kHz, 30 kHz, 40 kHz,50 kHz, 75 kHz, 100 kHz, >100 kHz.

The method may include executing step (iii) after step (ii) yet beforesumming a plurality of beamformed signals.

The method may include downshifting the frequency of the signal duringstep (ii), typically from the order of 1 GHz to the order of 50 MHz. Themethod may include providing ADCs to perform step (iv).

According to a tenth aspect of the present invention there is provided aprogram storage device readable by a machine and encoding a program ofinstructions which when operated upon the machine cause it to operate asthe system according to any one of the first, second third, fourth, oreighth aspects of the present invention.

According to a eleventh aspect of the present invention there isprovided a computer readable medium having stored therein instructionsfor causing a device to execute the method of the fifth, sixth or ninthaspects of the present invention.

According to a twelfth aspect of the present invention there is provideda telecommunications system incorporating an adaptive signal processingsystem according to any one of the first, second, third, fourth, oreighth aspects of the present invention and/or being arranged to executethe method of either of the fifth, sixth or ninth aspects of the presentinvention.

According to an thirteenth aspect of the present invention there isprovided a method of increasing the number of users accommodated withina telecommunications channel of a given bandwidth by spatially filteringsignals arriving at a receiver using selectively an adaptive signalprocessing system according to any one of the first, second, third,fourth or eighth aspects of the present invention, and/or executing themethod of either of the fifth, sixth or ninth aspects of the presentinvention.

According to another aspect of the present invention there is providedan adaptive signal processing system comprising a plurality of receivingelements, a plurality of analogue to digital converters (ADC's) anddigital signal processing means; each of the receiving elements having arespective one of the ADC's connected thereto and being arranged toreceive a respective incoming signal, wherein each ADC is arranged toconvert a first portion of the respective incoming signals into adigital form at a sampling rate that is less than the temporal Nyquistrate of the incoming signal, and the signal processing means is arrangedto calculate complex weighting coefficients to be applied to respectivesecond portions of the respective incoming signals.

According to yet another aspect of the present invention there isprovided a method of producing weighting coefficients for adaptivebeamforming comprising undersampling signals from antennae elements incomparison with a temporal Nyquist rate; and using the undersampledsignals to produce weighting coefficients for each antenna element.

According to a further aspect of the present invention, there isprovided an adaptive signal processing system comprising a plurality ofreceiving elements, beamforming means, a plurality of analogue todigital converters (ADCs), digital processing means, and in which thesignal weighting means have a plurality of input channels and arespective plurality of output channels, the signal processing meansincluding a memory unit arranged to temporarily store a plurality ofpreviously calculated complex weighting coefficients, each of thereceiving elements being arranged to receive an incoming signal andbeing connected to a respective input channel, each of the ADCs beingarranged to sample an analogue signal from an output channel, andconvert it into digital signals wherein the processing means is arrangedto calculate new complex weighting coefficients using the digitisedsignals and the previously calculated coefficients.

According to a yet further aspect of the present invention, there isprovided a method of adaptive signal processing comprising the steps of:

-   -   (i) receiving an analogue signal;    -   (ii) beamforming the signal;    -   (iii) sampling the signal after beamforming;    -   (iv) converting the signal sample into a digital signal; and    -   (v) calculating a new complex weighting coefficient using the        digitised signal; using the digital signal and the previously        calculated complex weighting coefficients digitally.

The invention will now be described by way of example only, withreference to the accompanying drawings, in which:

FIG. 1 is a schematic representation of a prior art phased array antennadevice with beam former;

FIG. 2 is a representation of a wavefront impinging upon two elements inan array aperture of the device of FIG. 1;

FIG. 3 is a schematic representation of a prior art analogue smartphased array antenna;

FIG. 3 a is a schematic representation of a signal coupling arrangement;

FIG. 4 is a schematic representation of a prior art analogue beamsteering adaptive phased array antenna;

FIG. 4 a is a further representation of a prior art hybrid digitallycontrolled analogue adaptive phased array antenna;

FIG. 5 is a schematic representation of a temporal Nyquist sampling ratefully digital adaptive phased array antenna;

FIG. 6 is a schematic representation of an analogue adaptive phasedarray antenna;

FIG. 7 is a schematic representation of a fully digital undersamplingadaptive phased array antenna according to an aspect of the presentinvention;

FIG. 8 is a schematic representation of a first embodiment of adigitally sampled and controlled undersampled adaptive phased arrayantenna according to an aspect of the present invention;

FIG. 8 a is a schematic representation of a further arrangement of anundersampled adaptive phased array antenna;

FIG. 9 is a schematic representation of a second embodiment of theundersampled adaptive phased array of FIG. 8;

FIG. 9 a is a schematic representation of a further arrangement of anundersampled adaptive phased array antenna;

FIG. 10 is a third embodiment of the undersampled adaptive phased arrayantenna of FIG. 8;

FIG. 11 is a schematic representation of a yet further embodiment of aphased array antenna according to at least one aspect of the presentinvention;

FIG. 12 is a flowchart detailing a method of signal processing accordingto an aspect of the present invention; and

FIG. 13 is a flowchart detailing a method of signal processing accordingto another aspect of the present invention.

Referring now to FIG. 1, a phased array device 100 comprises a pluralityof receiving elements 102 a-102 n each having a respective one ofweighting units 104 a-104 n associated therewith and a beamformer 106.The receiving elements define an array aperture 108, a fraction ofradiation impinging upon the array aperture 108 is detected by each ofthe receiving elements 102 a-102 n. This situation is shown in detail inFIG. 2, in which parts similar to those of FIG. 1 are accorded similarreference numerals in the two hundred series.

Referring now to FIG. 2, a wavefront 210 is incident upon the arrayaperture 208, at an angle θ to the normal of the aperture 208, along avector A-A. Considering two adjacent receiving elements 202 a,b thewavefront 210 must travel an additional distance y after being receivedby the element 202 b before being received by the element 202 a. Fromsimple geometric consideration it can be seen that y=d sin θ. This extradistance of travel introduces a phaseshift between the wavefrontreceived at the two elements 20 a,b of$Ø = {\left( \frac{2\pi}{\lambda} \right)d\quad\sin\quad\theta}$sin θ.

The weighting units 208 a,b apply a correction in order that theelectric vectors of the respective fractions of the wavefront detectedat the receiving elements 202 a,b are aligned prior to passing to thebeamformer 206 to be summed. Thus, it can be seen that by altering thephase correction applied to the detected fractions of the wavefront 210the antenna array can be spatially scanned as each direction willexhibit a unique phase relationship between the receiving elements 202a-n.

Also it can be seen that directions can be spatially filtered byweighting the phase corrections to the receiving elements such that thesum of the fractions of the E vector of a wavefront from the desireddirections detected by the receiving elements is zero. This is achievedby weighting the phase corrections such that the fractions of the Evector detected are in antiphase to each other and therefore sum tozero.

Whilst nulling increases the number of radiators that can beaccommodated within a channel, by spatially selecting those radiators towhich the channel is open (or deselecting radiators not to beconsidered), only a limited number of directions can be excluded fromany given channel. The number of directions that can be excluded from achannel is determined by the number of degrees of freedom of the sensorarray, for a sensor array having n elements there are n−1 degrees offreedom that can be nulled.

Referring now to FIG. 3, this shows a smart antenna arrangement 300 asused in telecommunications applications. The antenna arrangementcomprises a plurality of receiving elements 302 a-n each with anassociated weighting units 304 a-n, a beamsformer 306, an adaptiveprocessing unit 308 and a difference unit 310.

Instead of/in addition to nulling, other adaptive beamforming techniquescan be applied to minimise problems from sidelobes.

The receiving elements 302 a-n, weighting unit 304 a-n and beamsformer306 operate substantially in the fashion described hereinbefore.However, a portion of the signal 312 a-n received by each of receivingelements is tapped off, or coupled to a sampling arrangement, see forexample FIG. 3 a, prior to the signal passing through the weightingunits 304 a-n, and is passed to the adaptive processing unit 308.

The beamformer 306 outputs a signal 314, which has a sample 316 tappedoff and passed to the difference unit 310, where it is compared to atraining signal 317. The training signal 317 is an estimate, of if thedesired output is known it is, the output signal desired from thebeamformer 306. An error signal 318 is generated by the difference unit310 based upon the difference between the sample 316 and the trainingsignal 317 which is passed to the adaptive processing unit 308.

The adaptive processing unit 308 attempts to minimise the differencebetween the sample 316 and the training signal 317 by carrying out aleast squares minimisation analysis varying weights 320 a-n sent to eachof the weighting units 304 a-n in order to achieve this.

This arrangement has the problem that the desired (training) signal 317may only be known for a short time due to the low temporal coherencetime of non-static radiators.

Referring now to FIG. 3 a, a signal coupling arrangement 350 comprises alocal oscillator (LO) 352, a coupling region 354, a signal pick up 355,an amplifier 356, an ADC 358 and an adaptive processor 360. The ADC 358includes a sample-and-hold circuit 362, an ADC unit 364 and a clockinput 366. The adaptive processor 360 includes an input 368, a signalprocessor 370, a memory unit 372 and an output 374.

A signal passes from the LO 352 to the coupling region 354 where thepick up 355 is, typically, inductively coupled to the coupling region354. The coupled signal is then amplified by the amplifier 356 andpasses to the ADC 358. The digitised signal is then passed to the input368 of the adaptive processor 360 and is processed by the signalprocessor 370 in order to calculate a complex weighting coefficient forthe signal.

The memory unit 372 allows previously calculated weighting coefficientsto be stored in order that they can be removed from the coupled signalprior to the calculation of new weighting coefficients should thecoupling region 354 be situated after a beamformer (not shown).

The calculated adaptive complex weighting coefficients are passed viathe output 374 to a beamformer.

Referring now to FIG. 4, this shows an adaptive beam steeringarrangement 400 which is substantially similar to the smart antennaarrangement of FIG. 3 except that a steering vector 401 is directlyapplied to an adaptive processing unit 408 as the weights 420 a-nrequired to sight in a given direction are known and will typically bestored in the processing unit 408. The beamformer output 414 isprocessed subject to a power minimisation constraint in order to reduce,in far as is practicable, the effects of interfering radiators (alsosubject to the constraint of the steering vector).

In all of the aforementioned arrangements the Nyquist sampling criteriafor the production of the weighting coefficients is not an issue as theyall employ analogue adaptive techniques. However, there is the problemassociated with all analogue techniques that the signals are more likelyto become corrupt than a digital signal and the signal processingtechniques are not as robust as digital processing techniques.

FIG. 4 a is a schematic diagram of a low cost phased array antenna 450comprising a plurality of receiver chains 452 a-n, a beamformer and asummation unit 456. Each receiver chain 452 a-n comprises a receivingelement 458, a low noise amplifier 460, an image reject filter 462 and afrequency downshifitng mixer 464.

Receiver chains 452 pass received signals to a digitally controlledanalogue beamformer 454 that operates at a down-shifted intermediatefrequency, typically around 1 GHz.

The output of the beamformer 454 is typically at around 50 MHz and ispassed to the summation unit 456 from where the beamformed output ispassed to external circuitry for further processing/analysis.

Referring now to FIG. 5, this shows a fully digital implementation of atemporal Nyquist sampling rate adaptive phased array antenna 500comprising a plurality of receiving elements 502 a-n each with anassociated high speed analogue to digital converter (ADC) 504 a-n, adigital microprocessor 506 and an adaptive processor 508.

Each of the receiving element 502 a-n receives a fraction of an incidentwavefront (not shown) as this passes down receiver chains 510 a-n it isdigitised by the high speed ADCs 504 a-n. A first portion of each of thedigitised signals 512 a-n is split or coupled to the adaptive processor508 and a second portion 514 a-n passes to the microprocessor.

An adaptive algorithm acts upon the first portion of the digitisedsignals 512 a-n in order to calculate the required weightings 515 a-nfor a desired steering vector 516 that is also input to the adaptiveprocessor 508.

The second portion of the digitised signals 514 a-n passes to themicroprocessor 506. The required weightings 515 a-n are also passed tothe microprocessor 506 where they act upon the second portion of thedigitised signals 514 a-n in order to phase correct them and themicroprocessor 506 performs the role of beamformer.

Although shown as separate devices it will be appreciated that themicroprocessor 506 and the adaptive processor 508 may be part of thesame digital signal processor or indeed may be signal processingalgorithms running on the same or different microprocessor(s)/digitalsignal processor(s).

This arrangement does provide for a complete digital architecture thatis capable of beamforming, however the temporal information rate upon anincoming channel is typically tens of MHz. Therefore in order to becapable of sampling the incoming channel in the temporal domain withoutthe possibility of aliasing the ADCs must be capable of satisfying thetemporal Nyquist criterion of sampling at twice the rate of the maximumbandwidth of the incoming signal, i.e. 2× tens of MHz. ADCs capable ofoperating at these sampling rates do exist but are very expensive. Theyneed to sample and hold signals at the very high frequencies, andconvert them to digital signals at the very high signal frequencies.

Referring now to FIG. 6, a temporal Nyquist sampling rate analogueadaptive array antenna 600 comprises a plurality of receiving elements602 a-n, a plurality of phase changers 604 a-n associated withrespective receiving elements 602 a-n and a beamformer 606. Each phasechanger 604 a-n comprises a control unit 608 and a vector modulator 610.The phase changers 604 a-n are intended to correct the phase of anincoming wavefront.

In all of the aforementioned arrangements standard beamformingtechniques such as tapering the output across the array aperture inorder to reduce sidelobe ripple and the use of attenuators in order tominimise sidelobe contributions can be employed.

As the prior art discussed hereinbefore show the temporal Nyquistsampling of high frequency, wide bandwidth information channels foradaptive antenna arrays is a non-trivial exercise and can only beaccomplished by the use of ultra-fast, high cost digital components andADCs or by highly complex analogue signal processing and circuitry.

However, sampling in the spatial domain provides an alternative samplingregime that has a lower Nyquist sampling rate constraint than thetemporal domain, typically 30-40 kHz cf. 5 MHz temporal bandwidth. Thisis because rate of the spatial variation in the environment is very muchlower than the rate of temporal variation. In order that the adaptivealgorithm functions effectively the spatial covariance of the arrivingsignal environment must be accurately estimated. For a low costnarrowband system this can be assumed to be frequency independent. Thetemporal signals may be undersampled with no loss of spatial covarianceinformation. i.e. by undersampling at under the Nyquist rate higherfrequencies are folded into the sampling bandwidth with their spatialcovariance information preserved. (A mathematical treatment of thecovariance in both the spatial and temporal domains can be found inAppendix A.)

This reduces the need for ultra-fast digital components and ADCs inorder to calculate the complex weighting components required forbeamforming. It is still necessary to use ultra-fast digital componentsor analogue circuitry for the analysis of the temporal information froman information channel. This is clearly the case for a digital analysis,as any such analysis must still fulfil the temporal Nyquist criteria inorder to prevent aliasing and loss of temporal information from thesignal.

Referring to FIG. 7, a digital, temporally undersampled, phased arrayantenna 700 comprises a plurality of receiving elements 702 a-n, aplurality of high sampling rate ADCs 704 a-n, a plurality of lowsampling rate ADCs 706 a-n, a microprocessor 708 and an adaptiveprocessor 710. Any given receiving element 702 a has a respective highsampling rate ADC 704 a and a low sampling rate ADC 706 a associatedtherewith.

Each receiving element 702 a-n receives a fraction of a wavefront (notshown) incident upon an array aperture 712 and forming respectivereceived signals 714 a-n. A first portion 716 a-n of each of thereceived signal 714 a-n is tapped off and fed to the low sampling rateADCs 706 a-n. The low sampling rate ADCs 706 a-n pass the digitalsignals to the adaptive processor 708 where the complex weightingcoefficients are calculated for an input steering vector 718.

A second portion 720 a-n of each of the received signals 714 a-n passesdirectly to the high sampling rate ADCs 704 a-n. The resultant digitalsignals are passed to the microprocessor 708 where they are processedalong with their respective complex weighting coefficients in order toperform the beamforming operation. This arrangement reduces thecomputational load upon the high speed microprocessor 708 and allows thecalculation of the complex weighting coefficients to be calculated bythe adaptive processor 706 which may have reduced computational powercompared to the microprocessor 708.

Despite having low sampling rates the ADCs 706 a-n must havesufficiently low sample-and-hold (S/H) times that the acquisition timesto be able to capture the incoming analogue signal. A typical radarinput signal will have a frequency of around 10 GHz. This requires a S/Hacquisition time of 0.1 ns or less, which is at the upper end of currentcapabilities. This requirement for very low S/H acquisition time ADCsincreases the cost of this arrangement but it is still below that offull temporal Nyquist sampling arrangements because of the reduction inthe computational power required, and because the actual conversion fromanalogue to digital does not need to be performed so fast—only the“snap-shot” capture of the signal. One possible solution would be to adda further mixer stage before digitising. Thus, the ADC input would be alower IF and thus the S/H specification could be relaxed.

Referring now to FIG. 8, a digitally controlled, temporallyundersampled, analogue phase array antenna 800 comprises a plurality ofreceiver channels 802 a-n, a beamformer 804, and plurality of lowsampling rate ADCs 806 a-n, and a digital adaptive processor 808.

Each receiver channel 802 a-n comprises a receiving element 810, adownshifting frequency mixer 812, a phase modulator 814 and anattenuator 816. The phase modulator 814 comprises a digital control unit818 and a vector modulator 820, typically a phase controlled localoscillator and diode arrangement.

A signal is received at the receiving element 810 and is frequencydownshifted by the mixer 812, typically an incoming radar signal with afrequency of 10 GHz is downshifted to 1 GHz at this stage.

A portion of the signal is tapped off after the mixer 812 and before thephase modulator 814. This portion of the signal passes to a respectiveADC 806 a-n. The digitised signal is then passed to the digital adaptiveprocessor 808 where the complex weighting coefficients for an inputsteering vector 822 are calculated.

The complex weighting coefficients for each receiver channel 802 a-n arepassed from the adaptive processor 808 to the digital control unit 818of the respective phase modulator 814 where they are converted intoanalogue signals and applied to the vector modulator 820.

The remainder of the incoming signal passes down the receiving chain 802a-n from the mixer 812 to the phase modulator 814 where the complexweighting coefficients are applied thereto.

The phase modulator 814 not only acts to impose the phase of the localoscillator upon the incoming signal, thereby effecting the weighting ofthe incoming signal, but typically also acts as a downshifting frequencymixer. In a typical arrangement the post mixer 812 1 GHz signal will bemixed down to 140 MHz by mixing it with an 860 MHz signal at the phasemodulator 814.

From the phase modulator 814 the signal passes through the attenuator816 to the beamformer 804 where analogue beamforming takes place in theknown manner, by the summation of the weighted contributions from eachof the receiving chains 802 a-n. The attenuator 816 acts to removesidelobe contributions from a radiator's output radiation pattern.

Whilst this arrangement still requires short acquisition time,comparatively expensive, SH circuits on the ADCs 806 a-n, as a 1 GHzsignal may, in some embodiments still be required to be acquired inapproximately 0.1 ns, it does have the advantage that the phasemodulator 814 has not altered the phase of the incoming sample prior toit's sampling. Therefore there is not the need to computationally removethese complex weightings from the sampled signal before calculating newcomplex weightings. Also, as the signal has undergone relatively minoranalogue signal processing the opportunities for distortion of thesignal, with the consequential loss of information content, areminimised.

Referring now to FIG. 9, similar parts to those shown in FIG. 8 areaccorded the same reference numeral in the nine hundred series, in thisarrangement, which is substantially the same as that of FIG. 8, theincoming signal is sampled between the phase modulator 914 and theattenuator 916.

As noted hereinbefore this does require the adaptive processor 908 tocomputationally remove the previous complex weighting coefficients fromthe sampled signal prior to calculating new complex weightingcoefficients. However, this can be overcome by storing the previous setof coefficients in a memory unit 901 of the processor 908 and callingthem from memory 901 when required in order to recreate the uncorruptedfrequency downshifted signal. This arrangement does not require suchhigh specification SH circuits within the ADCs 906 a-n as the incomingsignal has been further frequency downshifted by the phase modulator914, typically to about 140 MHz.

FIGS. 8 a and 9 a show alternative sampling arrangements for adaptivephased array antennae according to an aspect of the present invention.

Referring now to FIG. 8 a, a phased array antenna 850 comprises aplurality of receiving channels 852 a-n, a plurality of ADC's 854 a-n,an intermediate frequency beamformer 856, a digital adaptive processor858 and a summation unit 860. Each receiving channel 852 a-n comprises areceiving element 862, a low noise amplifier 864, an image reject fitter866 and a downshifting frequency mixer 868. The beamformer 856 istypically an array of digitally controlled phase modulators. Thesampling of an incoming signal, A/D conversion and calculation of thecomplex weighting coefficients is substantially as described in FIG. 8.

The complex weighting coefficients are passed to the beamformer 856,typically operating at 1 GHz, where the phase corrections are applied tothe incoming signals. Typically, the beamformer 856 also downshifts thefrequency of the signals further, for example to 50 MHz, prior to thesignals passing to the summation unit 860. The summation unit 860performs the summation of the signals to generate a single beamformedoutput.

Referring to FIG. 9 a, similar parts to those of FIG. 8 a will beaccorded the same reference numerals in the nine hundred series. In thisarrangement, the signals are sampled after they have passed through theintermediate beamformer 956. This necessitates the adaptive processor958 storing the previously applied complex weighting coefficients inorder that they can be subtracted from the sampled signal prior to thecalculation of a new set of complex weighting coefficients. Other thanthe differences noted above the arrangement of FIG. 9 a operatessubstantially as described hereinbefore in relation to the arrangementof FIG. 8 a.

Referring now to FIG. 10, similar parts to those shown in FIG. 8 areaccorded the same reference numeral in the one thousand series, in thisarrangement, which is substantially the same as that of FIG. 8, theincoming signal is sampled between the attenuator 1016 and thebeamformer 1004.

This arrangement requires the previous complex weighting coefficients tobe removed computationally using adaptive processor 1008 that stores theprevious complex weighting coefficients in memory unit 1001 prior to thecalculation of a new set of complex weighting coefficients, as describedin relation to the arrangement of FIG. 9.

Further to which this arrangement will exhibit a lower signal-to-noiseratio than the arrangements of FIGS. 9 and 10 due to having passedthrough the attenuator and may therefore suffer from some loss ofinformation within the signal. However, in applications wheresuppression of sidelobe contributions are important this arrangementwill have significant benefits.

This arrangement will be able to operate with the longer SH acquisitiontime ADCs 1006 a-n.

Thus, the embodiments of the invention of FIGS. 7, 8, 9 and 10 all havetheir own distinct merits relative to one another.

Referring now to FIG. 11, a phased array antenna 1100 comprises aplurality of receiving chains 1102 a-n, a plurality of ADC's 1106 a-n,an intermediate frequency beamformer 1104, a digital adaptive processor1108 and a summation unit 1110. Each receiving chain 1102 a-n comprisesa receiving element 1112, a low noise amplifier 1114, an image rejectfitter 1116 and a downshifting frequency mixer 1118. The beamformer 1106is typically an array of phase modulators 1104′a-n that are controlledby respective digital control units 1104″a-n. The adaptive processor1109 includes a memory unit 1119 that stores previously calculatedcomplex weighting coefficients.

Each receiving element 1112 a-n receives a portion of a wavefront (notshown). An analogue signal corresponding to a respective portion of thewavefront passes from the receiving elements 1112 a-n to theintermediate frequency beamformer 1104 via respective amplifiers 1114,filters 1116 and mixers 1118.

The beamformer 1104 applies complex weighting coefficients calculated bythe process or 1108 to each of the incoming signals, typically inresponse to a steering vector input into the processor 1108, in order tosteer an output beam for a response pattern of a receiving array. Thebeamformer 1104 usually operates at about 1 GHz. Typically thebeamformer 1104 downshifts the signals frequency to about 50 MHz priorto outputting the signals to the summation unit 1110.

Each of the signals exits the beamformer 1104 by respective outputchannels. Each signal is sampled after leaving the beamformer 1104 butbefore entering the summation unit 1110, typically either by a couplingarrangement, as shown in FIG. 3 a, or by tapping a portion of each ofthe signals. The samples of the analogue signals pass to respective ADCs1106 a-n where they are digitised.

The digitised signal samples pass to the adaptive processor 1108. Theprocessor 1108 accesses the previously calculated weighting coefficientsfrom the memory unit and uses them to modify the sampled signals suchthat the sampled signals correspond to the received signal beforebeamforming, typically by, for example by the multiplication of eachsignal by the reciprocal of the complex weight directly. Alternatively,the complex weighting due to the weighting circuitry can be removedeither by the multiplication of each element of the estimated complexspatial covariance matrix of the incoming signals by the reciprocal ofthe appropriate two complex weightings multiplied together, or by preand post-multiplication of the estimated complex spatial covariancematrix of the incoming signals by diagonal matrices of the appropriatecomplex reciprocal weightings.

The processor 1108 calculates a new set of complex weightingcoefficients based upon the incoming wavefront and an input steeringvector 1120. The newly calculated weighting coefficients replace thepreviously calculated weighting coefficients in the memory unit 1119.

Digital signals including the complex weighting coefficients are sent bythe processor 1108 to respective digital control units 1104″a-n of thebeamformer 1104. These digital signals are converted to analogue signalsto control respective phase modulations 1104′a-n in order to carry outbeamforming upon incoming signals.

Referring now to FIG. 12, this is a flowchart detailing a method ofadaptive signal processing according to an aspect of the presentinvention.

An adaptive antenna array receiving element receives an incoming signal(step 1200). The incoming signal will typically undergo initial analogueprocessing (step 1202), for example frequency mixing, typically fromaround 2 GHz (for a wireless LAN) or >60 GHz (for short range radar) toaround 1 GHz, phase modulation and/or attenuation.

The signal is beamformed using an analogue beamformer that typicallyconsists of a digitally controlled analogue amplitude and phasemodulator. The phase modulator modulates the phase of the signal byimposing the phase of a local oscillator upon the signal and steps thesignal frequency down (step 1204), typically to around 50 MHz. The phaseof the local oscillator is set by the digital control unit describedhereinafter.

The beamformed signal is sampled, usually either by coupling or tappingoff a portion of the sample (step 1206). The sampled signal undergoesanalogue to digital conversion (step 1208). The digitised signal isprocessed by the processor. This processing involves the processoraccessing previously a calculated complex weighting coefficient (step1210) for each input signal, and deweighting the digitised signal usingthe previously calculated coefficient (step 1212), for example by themultiplication of each signal by the reciprocal of the complex weightdirectly. Alternatively, the complex weighting due to the weightingcircuitry can be removed either by the multiplication of each element ofthe complex covariance matrix by the reciprocal of the appropriate twocomplex weightings multiplied together, or by pre andpost-multiplication of the covariance matrix by diagonal matrices of theappropriate complex reciprocal weightings.

The deweighted signals are used to calculate a new complex weightingcoefficient for each input channel (step 1214), typically in conjunctionwith an input steering vector.

The new coefficients replace the previously calculated coefficients inthe adaptive processors memory (step 1216).

The now complex weighting coefficient is passed to the digital controlunit of the input channel where it is converted into an appropriateanalogue signal (step 1218) to be applied to the phase modulator (step1220) in order to carry out the beamforming operation (step 1204).

The analogue signal from the output of the beamformer passes to asummation unit where it is summed (step 1222) and output for furtherprocessing (step 1224).

Referring now to FIG. 13, this is a flowchart detailing a method ofadaptive signal processing according to an aspect of the presentinvention.

An adaptive antenna array receives an incoming signal (step 1300). Theincoming signal may or may not undergo an initial analogue processingstep, for example mixing, phase modulation and/or attenuation. A sampleportion (first portion) of the signal is tapped off from the main body(second portion) of the signal (Step 1302) and is subject to analogue todigital conversion at a sub-temporal Nyquist sampling rate (step 1304).The digital first portion of the signal is used to digitally calculatethe complex weighting coefficients required to perform beamforming (Step1306). This calculation may include a correction for any previouslyapplied complex weighting coefficients present within the sampled firstportion of the signal.

If a fully digital approach is being applied to the adaptive beamformingthe main body of the signal is subject to analogue to digital conversionat a rate above the temporal Nyquist rate of the incoming signal (step1308), in order to preserve temporally sensitive information within thesignal. A digital processor then carries out a beamforming routine usingthe digitally calculated complex weighting coefficients and thedigitised main body of the signal (step 1310).

Should a hybrid analogue-digital approach be taken to the adaptivebeamforming operation the main body of the signal will typically besubject to analogue signal processing (step 1312). The digitallycalculated complex weighting coefficients are passed to a digitalcontrol unit for an analogue phase modulator (step 1314). The analoguephase modulator modulates the phase of the main body of the incomingsignal, typically by imposing the phase of a local oscillator upon theincoming signal (Step 1316). The phase of the local oscillator havingbeen set by the digital control unit based upon the digitised complexweighting coefficients. The main body of the incoming signal then passto an analogue beamformer for summation in the usual fashion (Step1318).

The major advantages of the signal processing systems and methodsaccording to the present invention are the removal of the necessity tohave high speed ADC's which are costly with the consequential decreasein the processing power required to calculate the complex weightingcoefficients to be applied to form a beam. This increases the utility ofadaptive beamformers opening up further fields of usage including thespatial filtering by directional nulling in mobile telecommunicationnetworks, or wireless LANS, for example IEEE802.11, HiperLan orBluetooth, to allow the selective rejection of users from a channel at agiven point in time thereby increasing the number of users that can behandled in any one information channel.

It will be appreciated that although only referring to the reception ofradiation due to the reversible traceability of radiation any one, orcombination, of the methods, or systems, described hereinbefore may beapplied in a transmission mode as well as in reception mode.

Appendix A

It can be shown that the optimum solution for a weighting vector isgiven by:w_(opt)=R⁻¹C(C^(H)R⁻¹C)⁻¹f  (1)where C is the matrix of constraint vectors, f is a vector of gainconstraints and R is the matrix of cross-correlations between signalsarriving at the array aperture.

The primary limiting factor on the satisfactory performance of this typeof adaptive algorithm is the successful calculation of the covariancematrix R, since this is unknown and must be estimated from the receivedsignals.

The temporal signals may be undersampled with no loss of spatialcovariance information. i.e. by undersampling at under the Nyquist ratehigher frequencies are folded into the sampling bandwidth with theirspatial covariance information preserved.

A further constraint on the sampling rate is the rate at which thesignal environment changes. If the sampling rate is low then the up-daterate of the covariance matrix will be slow. Hence the covarianceestimate will be inaccurate for a signal environment which is movingquickly and the adaptive algorithm's performance degraded. For manyarray applications, however, the scenario changes relatively slowly.

If the signal vector entering the complex weighting circuitry (vectormodulator and digital attenuator) is given by the vector:x=(X₀x₁ . . . x_(n))^(T)  (2)then the calculated covariance matrix is given by:R=E[xx^(H)]  (3)

Where E[ ] is the expectation operator. In expanded format:$\begin{matrix}{R = \begin{bmatrix}\overset{\_}{x_{o}x_{o}^{*}} & \overset{\_}{x_{o}x_{1}^{*}} & \cdots & \overset{\_}{x_{o}x_{N}^{*}} \\\overset{\_}{x_{1}x_{o}^{*}} & \overset{\_}{x_{1}x_{1}^{*}} & \ldots & \overset{\_}{x_{1N}x_{N}^{*}} \\\vdots & ⋰ & ⋰ & \vdots \\\vdots & \quad & \cdots & \overset{\_}{x_{N}x_{N}^{*}}\end{bmatrix}} & (4)\end{matrix}$where * is the complex conjugate.

If the weighted signal vector is given by:x_(w)=(w₀ ^(*)x₀w₁ ^(*)x₁ . . . w_(N) ^(*)x_(N))^(T)  (5)then the covariance matrix, Rh, calculated from the pre-weighted signalvector is given by: $\begin{matrix}{R = \begin{bmatrix}{w_{0}^{*}w_{0}\overset{\_}{x_{0}x_{0}^{*}}} & {w_{0}^{*}w_{1}\overset{\_}{x_{0}x_{1}^{*}}} & \cdots & {w_{0}^{*}w_{N}\overset{\_}{x_{0}x_{N}^{*}}} \\{w_{1}^{*}w_{0}\overset{\_}{x_{1}x_{0}^{*}}} & {w_{1}^{*}w_{1}\overset{\_}{x_{1}x_{1}^{*}}} & \ldots & {w_{1}^{*}w_{N}\overset{\_}{x_{1N}x_{N}^{*}}} \\\vdots & ⋰ & ⋰ & \vdots \\\vdots & \quad & \cdots & {w_{N}^{*}w_{N}\overset{\_}{x_{N}x_{N}^{*}}}\end{bmatrix}} & (6)\end{matrix}$

This is a weighted version of the desired covariance matrix R. Since thearray weights are known, an estimate of the covariance matrix can bededuced from equation (6).

An adaptive weight calculation algorithm is discussed here in moredetail.

The simplest solution to the power minimisation problem is given byequation (1), repeated here for a single look direction unity gainconstraint:w=R⁻¹s(s^(H)R⁻¹s)⁻¹  (7)where s is a single look direction constraint, and R is the covariancematrix.

The conventional power minimisation adaptive algorithm attempts toreduce the total power in the beamformed output. Let us consider thecase where no attempt is made to remove the clutter from the adaptiveprocessing signal path.

The covariance matrix may be therefore be written as the sum of threecomponents:R=M+V+Q  (8)where M is the covariance of the interference, V is the covariance ofthe clutter and Q is the thermal noise covariance. Each is assumedmutually uncorrelated.

The adaptive weight vector solution of equation (7) would attempt tominimise the total power entering the array.the coupling corrected low sidelobe weight vector is defined as wq. Ifthe adapted beam pattern is described by a weight.vector w, then thetotal error power entering the beamformed output, i.e. the differencebetween the desired power and actual power, is given by: $\begin{matrix}{e = {{{k^{2}\left( {w - w_{q}} \right)}^{H}\begin{bmatrix}\frac{\pi}{2} & \quad & \quad \\{\int h} & {(\theta){g(\theta)}{s(\theta)}s^{H}} & {(\theta)d\quad\theta} \\\frac{- \pi}{2} & \quad & \quad\end{bmatrix}}\left( {w - {wq}} \right)}} & (10)\end{matrix}$where k²h(θ) is a weighting function which can be chosen to emphasiseparticular parts of the array pattern, g(θ) is the vector of elementpattern gains for angle θ and s(θ) is the steer vector for angle θ.

Since the term in square brackets is the weighted cross-correlation ofsignals entering. each element of the array, from all look angles, it isan N×N matrix, where N is the number of elements. This matrix is denotedas Z. It is identical to the space covariance of the clutter covarianceV except for the weighting factor k²h(θ) which acts to emphasise thesidelobe region in preference to the main beam region. Thus, by ajudicious choice of k and h(θ) an error power can be calculated that isdominated by the sidelobe rather than main beam clutter error power.Finding a weight vector that minimises this error power will reduce thegain in the sidelobe regions.

A suitable cost function to be minimised is the total power plus errorpower, given by:w^(H)+k²(w−w_(q))^(H)Z(w−w_(q))+λZ(c^(H)w−1)*(c^(H)w−1)  (11)where a look direction constraint of the form cHw=1 has been imposed inthe usual way. The second term in this equation is the error powercovariance whose dominance is under the control of parameter k. Thesolution to this minimisation problem is given by:w_(opt−[R+k) ²Z+λ²cc^(H)]⁻¹[k²Zw_(q)+λ²c]  (12)

Putting A=R+k2Z and A._(;-∞), it can be show that the solution to (12)under these conditions is: $\begin{matrix}{W_{opt} = {\frac{A^{- 1}c}{c^{H}A^{- 1}c} + {k^{2}A^{- 1}{Zw}_{q}} - \frac{k^{2}A^{- 1}{cc}^{H}A^{- 1}{Zw}_{q}}{c^{H}A^{- 1}c}}} & (13)\end{matrix}$

Thus, this weight vector will produce a beam which is arbitrarily closeto the quiescent weight vector wq, under the control of k, whilstsatisfying the look direction constraint and minimising the interferencepower.

In the limit, as k−0, the solution reduces to the form: $\begin{matrix}{W_{opt} = \frac{'R^{- 1}c}{c^{H}R^{- 1}c}} & (14)\end{matrix}$which is the well known solution for an array with a single constraintexplored in the previous section.

1. An adaptive signal processing system comprising a plurality ofreceiving elements, a plurality of analogue to digital converters(ADC's) and digital signal processing means; each of the receivingelements having a respective one of the ADC's connected thereto andbeing arranged to receive a respective incoming signal, wherein each ADCis arranged to convert a first portion of the respective incomingsignals into a digital form at a sampling rate that is less than thetemporal Nyquist rate of the incoming signal, and the signal processingmeans is arranged to calculate complex weighting co-efficients to beapplied to respective second portions of the respective incomingsignals.
 2. A system according to claim 1 wherein the system includes asecond plurality of ADC's that are arranged to sample the second portionof the respective incoming signals at at least the temporal Nyquistsampling rate.
 3. A system according to claim 2 wherein the firstplurality of ADC's are arranged to sample the incoming signals prior tothem reaching the second plurality of ADC's.
 4. A system according toclaim 1 wherein the system includes at least one plurality of phasechanging means each of which is connected to a digital control unit,each phase changing means being associated with a respective receivingelement.
 5. A system according to claim 4 wherein each control unit isarranged to supply a digital or an analogue signal to a respective phasechanging means such that the second portion of each respective incomingsignal has its phase and/or amplitude varied by an amount correspondingto the complex weighting coefficient.
 6. A system according to claim 4wherein the first plurality of ADC's are arranged to sample respectivesignals after they have been passed through the phase changing means. 7.A system according to claim 6 wherein the processing means may bearranged to remove the phase and/or amplitude variations imposed uponthe first portions of the respective signals by the phase changingmeans.
 8. A system according to claim 1 wherein the processing means arearranged to apply the complex weighting coefficients to the secondportion of the incoming signal.
 9. An adaptive signal processing systemcomprising a plurality of receiving elements, signal weighting means, aplurality of analogue to digital converters (ADCs), digital processingmeans, and in which the signal weighting means have a plurality of inputchannels and a respective plurality of output channels, the signalprocessing means including a memory unit arranged to temporarily store aplurality of previously calculated complex weighting coefficients, eachof the receiving elements being arranged to receive an incoming signaland being connected to a respective input channel, each of the ADCsbeing arranged to sample an analogue signal directly from an outputchannel, and convert it into digital signals wherein the processingmeans is arranged to calculate new complex weighting coefficients usingthe digitised signals and the previously calculated coefficients.
 10. Asystem according to claim 9 wherein the processing means is arranged totransfer the new coefficients to the signal weighting means and thesignal weighting means is arranged to apply the new coefficients to anincoming signal.
 11. A system according to claim 9 wherein the memoryunit is arranged to temporarily store the new complex weightingcoefficients by overwriting the previously calculated complex weightingcoefficients.
 12. A system according to claim 9 wherein the signalweighting means includes a plurality of digital control units and/orrespective analogue amplitude and phase modulators.
 13. A systemaccording to claim 12 wherein the digital control units are arranged toreceive respective new weighting coefficients from the processing meansand are arranged to control respective phase modulators so as tobeamform the incoming signals, in response to the new weightingcoefficients, in use.
 14. A method of producing weighting coefficientsfor adaptive beamforming comprising undersampling analogue signals fromantenna elements in comparison with a temporal Nyquist rate; convertingthe undersampled analogue signals to digital signals in order to producethe weighting coefficients.
 15. The method of claim 14 includingreceiving signals containing temporal information at a control processorat a first frequency and receiving signals relating to the weightingcoefficients at the control processor at a second frequency, the firstfrequency being significantly higher than the second frequency.
 16. Themethod of claim 14 including generating the weighting coefficients usingdigitally undersampled signals in order to control analogue combinationmeans to combine temporal signals with weighting coefficients
 17. Amethod of adaptive signal processing comprising the steps of: (i)receiving an analogue signal; (ii) downshifting the frequency of thesignal (iii) beamforming the signal; (iv) sampling the signal afterbeamforming; (v) converting the signal sample into a digital signal; and(vi) calculating a new complex weighting coefficient using the digitisedsignal; using the digital signal and the previously calculated complexweighting coefficients digitally.
 18. The method of claim 17 includingcontrolling step (ii) using the previously calculated complex weightingcoefficient.
 19. The method of claim 17 including receiving a pluralityof signals and executing steps (ii) to (v) for each respective signal.20. The method of claim 19 including executing steps (ii) to (v) inparallel for the plurality of signals.
 21. The method of claim 17including generating an analogue output from a digital control unit soas to control a phase modulator to execute step (ii).
 22. The method ofclaim 17 including accessing the previously calculated complex weightingcoefficient from a memory unit in order to perform step (v).
 23. Themethod of claims 17 including storing the new complex weightingcoefficient in the memory unit, overwriting the previously calculatedcoefficient with the new coefficients.
 24. The method of claim 17including processing the analogue signal prior to beamforming.
 25. Themethod of claim 24 wherein the processing includes any one, orcombination of the following: downshifting frequency mixing, attenuationand/or phase modulation.